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Spatio-temporal changes in river bank mass failures in the Lockyer Valley,

Queensland, Australia

Chris Thompson a,b,⁎, Jacky Croke b, James Grove c, Giri Khanal d

a Centre for Integrated Catchment Assessment and Management (ICAM), Australian National University, ACT 0200, Australiab Australian Rivers Institute, Grif th University, Nathan Campus, Queensland 4111, Australiac Department of Resource Management and Geography, University of Melbourne, VIC 3010, Australiad Department of Environment and Resource Management, Land Centre, Woolloongabba, Queensland 4102, Australia

a b s t r a c ta r t i c l e i n f o

Article history:

Received 31 December 2012

Received in revised form 31 January 2013

Accepted 12 March 2013

Available online 20 March 2013

Keywords:

Bank erosion

Mass failures

Exltration

Wet ows

Multitemporal LiDAR

Wet-ow river bank failureprocesses are poorlyunderstood relative to themore commonly studied processesof

uvial entrainment and gravity-induced mass failures. Using high resolution topographic data (LiDAR) and near

coincident aerial photography, this study documents the downstream distribution of river bank mass failures

which occurred as a result of a catastrophic oodin the Lockyer Valley in January2011. In addition, thisdistribu-

tion is compared with wet ow mass failure features from previous large oods. The downstream analysis of

these two temporal data sets indicated that they occur across a range of river lengths, catchment areas, bank

heights and angles and do not appear to be scale-dependent or spatially restricted to certain downstream

zones. The downstream trends of each bank failure distribution show limited spatial overlap with only 17% of

wet ows common to both distributions. The modication of these features during the catastrophic ood of

January 2011 also indicated that such features tend to form at some ‘optimum’ shape andshow limited evidence

of subsequent enlargement even when ow and energy conditions within the banks and channel were high.

Elevation changes indicate that such features show evidence for inlling during subsequent oods. The preser-

vation of these features in the landscape for a period of at least 150 years suggests that the seepage processes

dominant in their initial formation appear to have limited rolein their continuing enlargement over time. No ev-

idence of gully extension or headwall retreat is evident. It is estimated that at least 12 inundation events wouldbe required to ll these failures based on the average net elevation change recorded for the 2011 event. Existing

conceptualmodels of downstreambank erosion process zones mayneedto consider a wider array of mass failure

processes to accommodate for wet ow failures.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Understanding the mechanisms and rates of bank erosion is

paramount to the successful management of aquatic ecosystems

and off-shore environments, especially as numerous studies now

point to bank erosion as the dominant contributor to issues of

water quality and river degradation (Grimshaw and Lewin, 1980;

Prosser et al., 2001; Simon et al., 2002). However, in spite of the

recognition of its importance, there remain surprisingly few studies

of downstream changes in bank erosion processes and rates within

individual basins to enable effective quantication of the timing

and spatial distribution of sediment delivery (Lawler et al., 1999).

At the basin scale most studiesare derived from analysis of cartographic

sources and aerial photographs (Lewin, 1977; Gilvear et al., 2000;

Winterbottom and Gilvear, 2000; Kemp, 2004).

The availability of high resolution topographic data provided by

Light-Detection And Ranging (LiDAR), combined with aerial photogra-

phy, has opened up the possibility of more accurate mapping of bank

erosion volumes and processes (Bowen and Waltermire, 2002; Jones

et al., 2007; Marcus and Fonstad, 2008). Grove et al. (2013) used this

technology to classify bank forms along 100 km of the Lockyer Valley

southeast Queensland (SEQ) to estimate the relative contribution of

both uvial entrainment and mass failure processes during a cata-

strophic ood in 2011. This study highlighted that whilst the individual

processes of sub-aerial, uvial entrainment and mass failure bank ero-

sion have been well-studied (Thorne, 1982; Lawler, 1992, 1993;

Abernethy and Rutherfurd, 1998; Couper and Maddock, 2001; Rinaldi

and Darby, 2007), wet ow failures are poorly understood within

existing conceptual models. In addition, there is limited understanding

of the effect of local, at-a-site properties on their formation and how the

form of these features changes in response to subsequent ood events

over time. As the dominant mechanism for their formation isow seep-

age from near-saturated banks (Grove et al., 2013), existing theories

Geomorphology 191 (2013) 129–141

⁎ Corresponding authorat: Centrefor IntegratedCatchment Assessment and Management

(ICAM), Australian National University, ACT 0200, Australia.

E-mail address: [email protected] (C. Thompson).

0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.geomorph.2013.03.010

Contents lists available at SciVerse ScienceDirect

Geomorphology

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / g e o m o r p h

http://dx.doi.org/10.1016/j.geomorph.2013.03.010http://dx.doi.org/10.1016/j.geomorph.2013.03.010http://dx.doi.org/10.1016/j.geomorph.2013.03.010mailto:[email protected]://dx.doi.org/10.1016/j.geomorph.2013.03.010http://www.sciencedirect.com/science/journal/0169555Xhttp://www.sciencedirect.com/science/journal/0169555Xhttp://dx.doi.org/10.1016/j.geomorph.2013.03.010mailto:[email protected]://dx.doi.org/10.1016/j.geomorph.2013.03.010

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and conceptual models may be inappropriate in explaining their down-

streamdistribution andthe forces actingto changetheirform over time.

Existing concepts used to quantify uvial entrainment estimates

of bank erosion have focused primarily on principles of force and re-

sistance, where force is commonly measured using some expression

of energy or shear stress and resistance is reected in variations to

bank material properties and vegetation. Mass failure processes are

more spatially discrete, and are believed to be triggered by many fac-

tors such as pore water pressure, matrix-suction (Simon et al., 2002;Rinaldi et al., 2004) seepage forces (Rinaldi and Darby, 2007), ante-

cedent soil moisture condition (Hooke, 1979) and the force of gravity

(Thorne, 1993). In general, mass failures are thought to occur as the

shear strength of the soil is exceeded by the weight of the overlying

material when the hydraulic conductivity (Ks) of the river sediment

limits drainage so that water table cannot lower at the same rate as

the river stage (Dapporto et al., 2003). The timing, and potential, of

failure have traditionally been quantied using the factor of safety

(Fs) (Parker et al., 2008). Cohesive riverbanks have low Ks and the

ability to reach greater heights than other sediments, and so it has

been assumed that mass failure processes will be effective only

when a bank reaches a critical height (Thorne, 1982; Lawler, 1995).

The application of these models to wet-ow failures has yet to be

fully investigated.

Conceptual understanding of riverbank erosion at the basin scale

(Lawler, 1992, 1995; Lawler et al., 1999) tends to support the existence

of a generalised trend of sub aerial processes dominating the headwater

reaches, uvial processes in mid-basin reaches (Graf, 1982; Lawler,

1995) and mass failure processes in the downstream reaches (Lawler

et al., 1999). These processes are, however, not mutually exclusive

and interactions occur between the different process types (Darby et

al., 2007; Rinaldi and Darby, 2007). The extent to which any of these

conceptual models applies to wet-ow mass failure processes is largely

untested in river systems.

This study seeks to advance our understanding of wet-ow mass

failure features by addressing three specic aims. Firstly, this paper

aims to compare the 2011 spatial distribution of mass failures as

reported in Grove et al. (2013) with the distribution of pre-existing

mass failures. Secondly, the paper will investigate the role of local,at-a-site bank and hydraulic parameters in explaining the downstream

distribution of these features. Thirdly, this study will investigate tempo-

ral modication of these features by comparing net changes in bank

form and volume between the two time periods.

2. Study area

2.1. Regional setting

The Lockyer Valley lies inland from Brisbane and extends to the

Great Dividing Range which marks the catchment divide from the

Murray–Darling Basin (Fig. 1). The Lockyer catchment drains nearly

3000 km2 of prime agricultural land in southeast Queensland

(SEQ). SEQ is a subtropical region with mean maximum monthlytemperatures ranging between 21 and 29 °C. The total annual rain-

fall ranges between 900 and 1800 mm, with the majority falling dur-

ing the warm summer season (October to February). The region is

characterised by seasonally variable patterns of oods and droughts

which have been linked to the inter-annual rainfall variations of the

El Nino-Southern Oscillation (ENSO) and the Inter-decadal Pacic

Oscillation (IPO) (Kiem et al., 2003; Rustomji et al., 2009).

2.2. Lockyer Creek catchment

The upper reaches of Lockyer Creek, known as Murphy's Creek

ow east over Jurassic Marburg sandstones before becoming bedrock

conned within the older Helidon sandstone with a mean channel

bed slope of 0.006 m m−1

. Downstream of the conned reaches,

channel gradient reduces to 0.0008 m m−1 as the river ows back

over the Marburg sandstones and discharges onto the unconned al-

luvial plain around Helidon, where the present low-ow channel is

inset within a large ‘macrochannel ’ (~150 m wide and 20 m deep)

containing within-channel benches. In the lower part of the catch-

ment where valley oor width is extensive (2–13 km), channel plan-

form alternates between low sinuosity reaches and tight meandering

bends which have incised into the Marburg sandstone. Some alluvial

cutoffs are preserved but there is little topographical evidence of re-cent lateral migration of the river in the form of remnant scroll bars

or extensive point-bar development. Levees are also notable features

with oodplain surfaces sloping steeply away from the present chan-

nel (Fig. 1D).

Early settlement commenced in the 1840s in the Lockyer Valley

with approximately half of the native vegetation in the region now

cleared, the majority of this occurring between 1840s–1940s

(Galbraith, 2009). The clearing mainly focused on the lower half of

the valley across its broad alluvial plainswhilst the steep upper catch-

ment remained largely uncleared (Galbraith, 2009). The density and

preservation of riparian vegetation adjacent to the macrochannel

are variable with the majority of bank tops largely devoid of wooded

vegetation as cropping land extends to the channel boundary.

2.3. Flood history

The January 2011 event is ranked as the second highest ood in

the past 100 years, after 1974 (Bureau of Meteorology, 2011). Major

ood events have also been documented in the Brisbane River in

the 1840s, 1890s, 1974 and 2011 (Table 1; Babister and Retallick,

2011). Hydrological records are scarce during the 1800s, however

historical documents and local newspapers refer back to the ‘great

ood’ of 1893 and the more recent ood of 1974 ood as the only

other major events prior to the 2011 ood. The ood in the 1840s is

likely to have coincided with rst settlement in the region and

would have encountered pre-European vegetation along the channel.

Riparian vegetation is likely to have been largely intact within the

macrochannel at the time of the 1893 oods.

3. Methods

This study is concerned with the spatial and temporal distributions

of riverbank mass failure features both as a result of the January 2011

ood and pre-existing oods. Theprimary data sources for the mapping

of these features were a combination of LiDAR and near-coincident

high-resolution aerial photography which were available for two time

periods; pre-ood (2010) and immediately after the January 2011

ood. Based on the combined extent of LiDAR coverage and post-ood

high resolution air photos, 71 km of stream length extending from

just above the township of Murphy's Creek to below Gatton (Fig. 1B,

C) was selected for analysis.

High-resolution DEMs of both time periods were constructed with

triangular irregular network (TINs) using Delaunay triangulation.Error and uncertainty in both DEM surfaces were investigated and

accounted for using procedures outlined in Croke et al. (2013).

3.1. Mass failure identi cation

A polygon layer was created of mass failures by tracing around the

headwall on the 2011 aerial imagery, guided by the >35° slope layer

derived fromthe LiDAR DEM to denethe 2011 post-ood mass failures

(Fig. 2). The steep slope was used to aid the visual interpretation of

eroding surfaces where there was canopy cover, and the 35° threshold

chosen is coincident with values of eroding banks reported by

Dapporto et al. (2003). The delineation of polygons was undertaken at

a scale of 1:400, as this was considered the optimum level to avoid pix-

ilation but maintain detail. During this mapping phase, detailed notes

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headwall was clearly identiable and a similar approach of applying

a >35° slope layer was used to guide the mapping. Pre-ood mass

failure length and width attributes were extracted whilst bank

height and angle were not extracted due to an absence of compara-

ble resolution elevation data for the likely failure time. Further,

there is also the probability of some modication of form over time

since these features were rst formed. Attempts to separate the

pre-ood features into discrete time steps using analysis of tradi-

tional black and white air photographs proved problematic. Selected

time periods for this preliminary analysis included time periods cov-

ering 1958, 1971 and 1974. Aerial photographs were scanned and

orthorectied to facilitate comparison between these data sources

and the pre-ood and post-ood LiDAR mapped features. As illus-

trated in a representative example of the 1974 air photo coverage

for a section of the river which experienced signicant mass failure

occurrences in 2011 (Fig. 3C), resolution proved too coarse to con-

dently interpret the existence of mass failures within this pre-ood

time period and the data set remained temporally lumped into apre-ood distribution.

3.2. Distribution of failures within hydraulically de ned geomorphic features

To determine where these features were forming, a geomorphic

classication of within-channel and oodplain features developed

using the LiDAR-derived DEM and a one dimensional (1-D) step back-

water model HEC-RAS was overlaid on the mass failure shape le. A

combination of discharge modelling and slope thresholds was used

to identify: (1) inner channel bed and bars; (2) inner channel

banks; (3) within-channel benches; (4) macrochannel banks; and

(5) inundated oodplain based on hydraulic modelling and terrain

slope thresholds (Table 3) (Croke et al., 2013).

3.3. Estimates of mass failure volume

Estimates of mass failure length, width and area for each time

period (pre- and post-ood) were extracted from each digitised

mass failure polygon. Net volumetric change between mass failure

features from the two time periods was approximated using a sim-

ple integration scheme, multiplying the calculated elevation change

(a depth measurement in m) from the DoD by surface area of each

cell (1 m2).

4. Results

4.1. 2011 and pre-existing mass failures

4.1.1. January 2011 mass failures

A total of 437 mass failures, with an average area of 676 m2

(Table 4), were identied and digitised throughout the study area

as a result of the 2011 ood. The failures could be attributed directly

to the

ood as they manifested as erosion on the DoD. It was not pos-sible to attribute a particular failure mechanism for 15 of these due to

problems of shading, shadows, and image resolution. Based on their

morphological attributes, 422 failures were classied as wet ow

mass failures. Three main types were recognized: (1) Piping failures

(cf Jones, 2010) (n = 168), with a concentration of exltrating ow

in one location; (2) coalesced piping failures (n = 154), where either

several failures had merged, or the landward migration from seepage

had caused bifurcation of the failure (Dunne, 1980; Schumm et al.,

1995); and (3) sapping failures (n = 100) (cf. Hagerty, 1991)

where the seepage ow is over a more extensive area possibly due

to more permeable sand lens or conning impermeable clay layer

(Fox et al., 2006).

The area occupied by each of the hydraulically dened geomor-

phic features in the study area shows that 53% of mass failure area oc-

curs over the macrochannel banks, 33% on benches, and 10% across

the inner channel banks, and 3% from oodplain surfaces (Table 5).

The total area covered by 2011 ood mass failures within the study

area is 295,350 m2 whilst a net volume of 695,214 m3 of material

was eroded.

4.1.2. Pre- ood mass failures

A total of 234 mass failures with an average area of 421 m2 were

identied and digitised from pre-ood high resolution LiDAR DEM

(Table 4). The mass failures that were evident in the earlier imagery

had the dimensions and morphology of single piping failures, with a

mean length: width ratio of 1.5 (±0.4), and not sapping failures.

An analysis of the spatial distribution of pre-existing mass failure

distribution shows that 60% of their area occurs over the macrochannelbanks, 21% on benches, 11% across theinner channel banks and 7% from

oodplains (Table 5). This distribution across hydraulically dened

geomorphic classes closely resembles the 2011 distribution. The

total area covered by pre-existing mass failures within the study

area is 98,508 m2, almost a third of the area compared to the recent

mass failures.

4.2. Spatial trends in post- and pre- ood mass failures

Post-ood mass failures rst occurred around 7 km downstream

of the Spring Bluff GS at a catchment area of 34 km2. The mass fail-

ures remained sparse until 38 km downstream (446 km2) at which

point the failure frequency dramatically increased to the down-

stream extent of the air imagery (Fig. 1C). A cumulative downstreamdistribution of mass failure area (Fig. 4) illustrates a stepped prole

with a hiatus in mass failures occurring between 30–40 km and

50–62 km, and the majority occurring below 62 km (1527 km2). A

Poisson distribution represents the distances between failures with

median distances of 45 and 44 m for the left and right banks respec-

tively (Fig. 5; Table 6). There was no signicant difference in dis-

tances between the two riverbanks.

Similar to the post-2011 distribution, pre-existing mass failures

occurred throughout most of the catchment, starting at 7 km down-

stream (Fig. 6), with a cluster of failures occurring around 30 km

Table 1

Major oods measured on the Lower Brisbane River (Port Of ce Gauge) unadjusted for

river changes and dam construction.

Flood year Flood height (AHD)† River changes affecting ood heights

1841 8.43 None

1844 7.02 None

1890 5.33 River mouth sand bar removed in 1864 &

ongoing river dredging for navigation

commenced

‡1893 5February

8.35 Ongoing dredging

19 February 8.09 Ongoing dredging

1898 5.02 Ongoing dredging to 1940s

‡1974 5.45 Flood mitigation works including river

widening commenced 1930s and

Somerset dam built in 1940s

‡2011 4.27 Wivenhoe Dam built in 1980s

† River heights are unadjusted for river changes.‡ Floods recorded in local newspapers impacting the Lockyer Valley.

Fig. 2. An exampleof river bankmass failures from theJanuary 2011oodshownwith (A)high resolutionair photo,(B) hillshade onLiDAR DEMand (C)air photowith overlayof

≥35° slope grid.


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B

A

C

133C. Thompson et al. / Geomorphology 191 (2013) 129–141

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and increased in frequency from 42 km downstream. Eighty ve

percent of the identied pre-ood mass failures occurred within

200 m of each other with the most isolated failure neighbour

being 2.4 km distant. A hiatus occurred between 54 and 62 km sim-

ilar to the gap in the recent mass failure distribution. A Poisson dis-

tribution represents the nearest neighbour distances with median

distances of 50 and 30 m for the left and right banks respectively

(Fig. 7; Table 5). There was no signicant difference in nearest

neighbour distance between both banks.

4.3. Temporal changes in patterns of mass failures

In spite of similar spatial distributions downstream, further anal-

yses revealed that only 17% of mass failures overlapped between the

two time periods (Table 4). Within these 75 coincident failures, 72%of the new failures had half or greater of their area within a

pre-existing failure, and less than 8% were completely located within a

pre-existing failure. On the other hand, of the intersecting pre-existing

failures, 19% were completely engulfed by the new 2011 mass failures.

In summary, of the total area occupied by mass failures (385,000 m 2),

only 2.3% is common to both pre-existing and 2011 mass failure

distributions.

4.3.1. Modi cation of existing failures during the 2011 event

Changes to the form of the existing mass failure distribution during

the 2011 ood event were calculated from the DoD. Overall, 137

pre-existing failures experienced net deposition whilst 97 had net ero-

sion (Fig. 8). The average amount of elevation change in the existingfailures is +0.08 m and only 2 of the 234 existing failures experienced

no erosion after the 2011 ood event. Modications of the pre-existing

mass failures consisted of three main process categories: (1) erosion at

the scarp of the failure; (2) erosion at the toe of failure; and (3) erosion

of the headland(s) outside of the failure. Scarp erosion, which may be

through both uvial entrainment and mass failure from seepage, was

evident at 32% of the failures. Toe erosion from uvial entrainment

was found at 39% of the sites, whilst headland erosion was found at

35% of the sites. Both net deposition and net erosion were evident in

all of these categories and at 95% of the failures overall. Vegetation,

which was visible after either erosion or deposition in the ood, was

signicant at 53% of the sites. Where vegetation was able to survive ero-

sion or deposition during the ood then the scarp erosion was mini-

mized from 32% to 12% and toe erosion from 39% to 21%. Only 12% of

the pre-existing failures had both signicant vegetation and scarp

erosion.

The erosion in and around the existing mass failures appeared to

exhibit the features of uvial entrainment. There were no large (in

relation to the failure size) discrete blocks on the failure oor, and

the DoD only showed change to a depth of 1–2 m around the scarp

face.

To investigate the effect of failure morphology on the form resis-

tance, ow velocity and erosion or deposition, the planform area of

the failure was correlated with the net DoD change of the failure.

Pre-existing failures that touched, or were contained by, 2011 failures

were excluded. The remaining 168 failures had a correlation coef cient

(R 2 = 0.40; n = 168) between area and net DoD change. If only the

failures that hadnet depositionare used to examine the relationship be-

tween planform area and deposition, the R 2 value increases to 0.70

(n = 114). The trend of increasing deposition with failure area appearsto weakenwith the larger failures, >2000 m2, and if these wereexclud-

ed from the correlation, the R 2 value rose to 0.78 (n = 108). This rela-

tionship did not appear to hold for the average elevation change in the

failure, with the 114 depositing sites only giving an R 2 value of −0.15.

So although the deposition increases as the failure size increases up to

2000 m2, the amount of deposition is not proportional to the failure

size.

4.4. Spatial distribution of mass failure site characteristics

The mean bank height on which mass failures occurred was 10.9 m,

though ranged from as lowas 4 m in the upper alluvial reaches to 19 m

in the mid Lockyer Valley (Table 7). Limited mass failures occurred onbanks over 15 m because banks of this height were restricted to a

short reach at 60–70 km downstream. Bank height shows a weak pos-

itive correlation against distance downstream (R 2 = 0.31), however

at the reach-scale bank height shows trends of both increasing and de-

creasing heightwith distance downstream (Fig. 9A).Bank slope showed

no correlation against distance downstream (R 2 = 0.03; Fig. 9B). Con-

tributing oodplain width increases exponentially downstream with a

notable step at ~70 km marking a widening of the valley oor down-

stream of Gatton (Fig. 9C).

Unit stream power (W m−2) showed a weak negative correlation

with distance downstream (R 2 = 0.06). In part, this weak trend is

due to a number of conned zones giving rise to peaks in unit stream

power (Fig. 9D). A peak in unit stream power in the upper conned

reaches of Murphy's Creek occurs at approximately 25 km

Table 2

Riverbank mass failure erosion types and their signature features on LiDAR and aerial imagery.

Mass failure type LiDAR last return identication features Aerial imagery identication features Source

Rotational

failure

• Arcuate vertical headwall.

• Failed blocks sloping away from the channel.

• Sharp break in vegetation at headwall.

• Exposed sediment at headwall.

• Large failure block with surface vegetation sloping away

from the channel.

Varnes (1978), Thorne (1982)

Slab failure • Linear steep headwall.

• High likelihood of a steep bank surface in precedingimagery.

• Blocks resting at the bank basal area.• The height of the block would be similar to the scarp

wall height.

• Failed blocks resting at bank toe, or resting on scarp wall.

• The failed block would have a relatively narrow

vegetated surface compared to its height.

Thorne (1982),

Dapporto et al. (2003)

Cantilever

failure

• Linear steep headwall.

• High likelihood of a steep bank surface in precedingimagery.

• Blocks resting at bank basal area smaller than

the scarp wall height.

• Failed blocks resting at bank toe.

• Failures could be as an elongated beam or block that has

been undercut and subsequently fallen either vertically,

or toppled forward. The relative position of the vegetated

surface would indicate the failure direction.

Thorne and Tovey (1981)

Wet ow • Arcuate scarp wall.

• A failureoorwith a smoothsurface,possibly incisedby ow.

• Concave failure surface.

• Possible narrow neck width compared to scarp

wall diameter.

• No obvious coherent blocks in the failure.

• Fluidised failed material would be expected to leave a

failure oor with ow

type features, possibly sinuous.

Wet sand, silt ow Varnes (1978)

Flow slide Hutchinson (1988)

Sand, silt ow slide

Hungr et al. (2001)


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B

C

A

Fig. 3. Images from same location in Fig. 2 with (A) air photo from 2009, (B) hillshade on 2010 LiDAR DEM showing mass failure scars and (C) a lower resolution 1971 air photo of

same reach.


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downstream (235 km2), and another increase mid-valley at approxi-

mately 50 km downstream.

4.5. Comparison between mass failure and non-mass failure site

characteristics

Bank height showed a signicant difference (p b 0.001) between

the mass failure and non-mass failure sites (Table 8). However,

non-mass failure locations are heavily biased by the limited number

of failures in the upper reaches of Murphy's Creek (Fig. 10A).

Re-analysis of bank height data excluding the upper 20 km, which

contained relatively few mass failures, resulted in no statistical differ-

ence between mass failure locations and non-mass failure locationswith mean bank heights of 11.1 m for both.

The unit stream power data showed a statistically signicant

result between the two populations based on a non-parametric

Wilcoxon test (p b 0.001) and an ANOVA on Log10 transformed data

(Table 8). The inclusion of non-mass failure sites in the higher energy

reaches of Murphy's creek, however, is also likely to have biased this

distribution, but as illustrated in Fig. 10B, the non-mass failure sites

display higher variation in unit stream power for the length of the

study area. Owing to the widespread distribution of the 437 mass fail-

ures in 2011, it proved problematic to accurately compare sites which

displayed no mass failures but nonetheless, ranges of variables are al-

most identical for the two data sets.

5. Discussion

5.1. Comparisons of failure form over time

This study presents unique, spatially comprehensive data on down-

stream trends in mass failure distributions as a result of a catastrophic

ood in January 2011 and those pre-existing failures in the Lockyer

Valley SEQ. The availability of high resolution LiDAR-derived DEMs

and aerial photography enabled accurate mapping of these features

from boththe post-ood and pre-ood data sets. The features described

in this study have been related primarily to wet ow processes which

form as liqueed or saturated material that is removed from the bank

face due to exltration and changes in pore water pressure when the

ood waters remain high (Grove et al., 2013). These were subse-

quently classied as piping and sapping failures in the 2011 distri-bution. Sapping failures had a more even backwall than piping

failures and appeared reasonably homogeneous in planform, with

less scalloping. The failure oor in sapping failures was often

stepped displaying multiple levelled/planar failure oors. Wet

ows caused by piping were the most consistent process through-

out the catchment in terms of the overall contribution of sediment

(Grove et al., 2013). Analysis of the pre-ood imagery conrms that

similar failures occurred during past oods as the accurate shape of

the headwall was clearly preserved, but less detail could be provid-

ed on their original form. Changes in the length/width ratios and

size of the features would indicate that sapping failures are less

dominant than identied in the post-ood data set. This could indi-

cate the general absence of this particular process or some subse-

quent change in the form of these features over time.

5.2. Controls on spatial trends in mass failure distribution

The downstream distributions of both pre- and post-ood failures

illustrate that they occur across a wide range of river lengths, catch-

ment areas and bank heights. The presence of such features in the

upper reaches just 7 km downstream in both distributions suggests

that such processes are not scale-dependent. Unlike the more widely

reported forms of mass failures due to rotational and planar processes

of bank collapse, wet ow failures occur fairly ubiquitously down-

stream, albeit with some areas displaying a general absence of fail-

ures. As a result, existing models for bank erosion process domains

which tend to conceptualise mass failures as occurring in the lower

reaches of valleys may not be appropriate for such features. Such

models are based largely on the assumption that as channel depth/ bank size increasesdownstream, a zone in which bank height exceeds

some ‘critical’ value leads to mass failure development downstream.

The bivariate plots of local at-a-site controls and distance down-

stream in this study illustrate some of the complexities of this gener-

alised interpretation in settings where such variables do not increase

linearly with distance downstream. For example, bank height shows a

linear trend of increasing for the rst 25 km of river length but then

shows a notable reduction, followed by several such ‘steps’ down-

stream. In this study, the presence of large ‘macrochannels ’ mid valley

in the Lockyer adds considerable complexity to any linear pattern of

increasing channel depth and bank size downstream. In addition, if

bank material properties or other channel geometry variables were

signicant factors in controlling the basin-scale distribution, existing

mass failures may be expected to enlarge over time. Analysis con-rmed that only 17% of post 2011 ood mass failures showed some

correlation to an existing feature indicating that the majority of

2011 failures occurred in a unique location. In addition, a comparison

of the changes in the form of pre-existing mass failures during the

2011 event would also lend support to the limited role of local,

at-a-site factors. There appears to have been limited modication of

the form of these features during the 2011 ood event. They did not

appear to contribute much sediment in the 2011 ood and were in-

stead operating primarily as sediment sinks. The correlation between

failure planform area and the amount of deposition showed a signi-

cant positive relationship between the existing failure size and the

net elevation change on the DoD, up to a threshold area of 2000 m2.

This could indicate that the larger failures have a limited ability to

create a dead water zone and perhaps do not form any ow

Table 3

Discharge and terrain slope thresholds used to distinguish geomorphic classes.

Geo-class Modelled discharge Terrain slope

Inner-channel bed and bars ≤Q 2.33 ≤10°

Inner-channel bank ≤Q 2.33 >10°

Bench >Q 2.33 and ≤Q bf ≤14°

Macro-channel bank >Q 2.33 and ≤Q bf >14°

Floodplain/terrace >Q bf ≤14°

Table 4

Number and area of digitised polygons in the study area representing pre-existing and

2011 mass failures, and number and area of intersecting polygons between the time

periods.

Number of MF Total area (m2) Average area (m2)

Pre-existing 234 98,508 421

2011 437 295,350 676

Overlapping MFs 75 9040 120

Table 5

Areal composition of geomorphic classes within the study area and the proportional

area occupied by the mass failures.

Geomorphic

class

Area

(ha)

Proportion

(%)

Area of pre-existing

mass failure (%)

Area of 2011 mass

failure (%)

Inner channel

bed and bars

102 1.3 2 1

Inner channel bank 106 1.4 11 10

Bench 206 2.7 21 33

Macrochannel bank 270 2.7 59 53Floodplain 6915 91.0 7 3


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separation, which would increase average velocity and decrease de-

position. As the basic mechanism for the formation of these features

is seepage from the oodplain face, it remains unclear why this pro-

cess does not continue to enlarge an existing mass failure or form a

gully in the headward extent of the failure wall. No gullies were

mapped adjacent to any pre-existing failure and none was formed

as a result of the 2011 ood. This would tend to add support to the

relatively limited role of local factors such as bank height or materialproperties in explaining their distribution.

5.3. Residence time of mass failure features

The preservation of mass failure features from past ood events is

an interesting conclusion emerging from this study. Comparison be-

tween the pre- and post-ood distributions would suggest that

mass failures from different oods tend to have limited spatial over-

lap and the overall form of the mass failure is retained in the land-

scape over time. The precise role of past historical oods in the

pre-ood mass failure distribution could not be fully elucidated, but

it seems likely that each of the major oods outlined in Section 2.3

contributed to the formation of these features, and as such they

have been preserved for at least 150 years. As the dominant process

occurring within an existing mass failure is now depositional, it

seems likely that such features will change their planform largely

through inlling and vertical accretion. This may explain the general

absence of sapping failures in the pre-ood distribution, as changes

in the length/width ratio may either reect the general absence of

this form in the initial distribution or subsequent modication of

0

10

20

30

40

50

60

70

80

90

100

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

0 10 20 30 40 50 60 70 80 90

C u m u l a t i v e m a s

s f a i l u r e a r e a ( % )

M a s s f a i l u r e a r e a ( m 2 )

Distance downstream (km)

Fig. 4. Longitudinal distribution of mass failures plotted against failure area from the January 2011 catastrophic ood.

0.0

0.5

1.0

1.5

2.0

2.5

0 10 20 30 40 50 60 70 80 90

N e a r e s t n e i g h b o u r ( k m )


Left bank

Right bank

Fig. 5. Longitudinal distribution of mass failures following the January 2011 catastrophic ood. Distance downstream commences from the Spring Bluff GS which has a contributing

area of 18 km2.

Table 6

Nearest neighbour distributions on left and right banks for pre-existing and 2011 ood

mass failures.

Year Bank Mean ± SD

(km)

Skewness Median

(km)

Min

(km)

Max

(km)

Pre-existing Left 0.15 ± 0.26 3.6 0.05 0 1.8

Right 0.15 ± 0.36 5.2 0.03 0 3.2

2011 Left 0.10 ± 0.22 7.1 0.045 0 2.1

Right 0.14 ± 0.31 4.9 0.044 0 2.4

Comparison Left 98 ± 226 7.1 0.044 0 2.1

Right 144 ± 315 4.9 0.044 0 2.4

No signicant difference in nearest neighbour distributions between year and bank

(p b 0.05) based on non-parametric test of medians and Kruskal–Wallis test.


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0

10

20

30

40

50

60

70

80

90

100

0

1000

2000

3000

4000

5000

6000

0 10 20 30 40 50 60 70 80 90

C u m u l a t i v e m a s s f a i l u r e a r e a ( % )

M a s s f a i l u

r e a r e a ( m 2 )

Distance down stream (km)

Fig. 6. Longitudinal distribution of mass failures plotted against failure area for pre-existing features.

0.0

0.5

1.0

1.5

2.0

2.5

0 10 20 30 40 50 60 70 80 90

N e a r e s t n e i g h b o u r ( k m )


Left bank

Right bank

Fig. 7. Longitudinal distribution of pre-existing mass failures.

-5000

0

5000

10000

15000

20000

25000

30000

35000

40000

0 10 20 30 40 50 60 70 80 90

M a s s f a i l u r e v o l u

m e ( m 3 )

Distance down stream (km)

2011 flood

pre-existing

Fig. 8. Change in sediment volume derived from 2010 to 2011 DoD. Positive values show sediment loss, negative values indicate in lling of mass failure areas.


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the failure form through depositional processes. It is estimated that at

least 12 inundation events would be required to ll these failures

based on the average net elevation change for the 168 failures in

2010 that did not touch the 2011 failures (0.13 m of deposition), as-

suming a constant rate of deposition in each event. In reality the

existing failures appeared to experience erosion at the toe or scarp

in 70% of the failures so that the failure margins would be more mod-

ied in the rst few events, which combined with deposition would

result in the smoothing of failure slopes. The degree of modication

initially would depend on time since the last event, controlling the

establishment of vegetation.

5.4. Mass failures and the magnitude of ood events

Whist the form of the pre-existing mass failures showed limited

change during the 2011 event, interestingly, the frequency of

post-ood mass failures is signicantly higher than the pre-ood dis-

tribution for a longer time period. The 2011 event saw an almost dou-

bling of mass failure features. This cannot be explained by ood

magnitude alone as the pre-ood time period also includes extreme

events which are reportedly of greater magnitude than the recent

2011 ood (Bureau of Meteorology, 2011). The 2011 event, for exam-

ple, is ranked as the second highest ood event in the last 100 years,

but as outlined in Section 2.3, historical accounts also reveal the sig-

nicance of earlier events such as 1840 and 1893 which based on

stage height data on adjacent rivers such as the Brisbane, were of

greater magnitude than both 1974 and 2011. The increase in the fre-

quency of the mass failures in the 2011 distribution, therefore, cannot

be explained by ood magnitude alone. However several aspects

of the 2011 ood are worth noting as they may have contributed

to this notable increase in the downstream reaches. The 2011

ood event occurred immediately following a very wet summer in

Queensland when the catchment and antecedent soil materials

were close to saturation even prior to the event (Bureau of

Meteorology, 2011; Jordan, 2011). The dominant source of rainfall oc-

curred in the upper catchment close to the headwaters near Murphy's

Creek, however due to the catchment morphology, the major tribu-

tary inputs on the southern margins of the catchment only ‘switched

on’ in the days following the major ood peak on January 10th. As

such, a double-peak was observed in the hydrograph at the lower

end of the system, which although of lower magnitude than up-

stream, would have contributed to more dynamic ow conditions

and pore-water pressure changes as the hydrograph adjusted

throughout both smaller events. The timing of the arrival of the rst

ood peak from the Lockyer is also known to have coincided with

the ood peak arrival down the mid-Brisbane such that ood waters

from the Lockyer were blocked at the tributary junction, increasing

the inundation time on the oodplains in the lower reaches. The effect

of bank inundation durations also increasing with ood-hydrographbase-times in a downstream direction has been noted in previous

Table 7

Characteristics of mass failures from January 2011 ood and associated channel attributes.

Variable Mean Std. dev. Median Skewness Min. Max.

Length (m) 46 65 24 5.0 5 757

Width (m) 16 11 13 2.9 3 105

Length/width 2.5 1.9 1.9 2.7 1 16.8

Area (m2) 676 1360 247 4.8 10 12,735

Volume (m3) 1592 3770 384 4.6 2 28,129

Bank height (m) 10.9 3 11 0.05 4 19

Bank slope (%) 34 13 32 0.02 5 84Contributing oodplain width (m) 772 810 505 1.8 10 3954

Unit stream power (W m−2) 189 227 130 4.8 1 2432

0

500

1,000

1,500

2,000

2,500

3,000

0 20 40 60 80

F l o o d p o w e r ( W m

- 2 )


0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80

B a n k s l o p e ( m / m )


0

1,000

2,000

3,000

4,000

5,000

0 20 40 60 80 C o n t r i b u t i n g f l o o d p l a i n

w i d t h ( m )


0

5

10

15

20

0 20 40 60 80

B a n k h e i g h t ( m )


A B

C D

Fig. 9. A comparison of 2011 mass failure characteristics and associated channel and hydraulic variables.


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studies of mass failure distributions downstream (Lawler et al.,

1999). Previous research also noted the existence of time-lags in-

volved in river banks reaching moisture-driven critical stability con-

ditions and as a result, many traditional mass failures have been

observed to occur on the recessional limbs of, or well after, storm

or seasonal hydrographs (Lawler et al., 1999). The general absence

of material on the oor of the failures post 2011 would also indicate

that such features occurred on the recessional limb, but when dis-charges remained high enough to transport much of the bank mate-

rial. Whilst this study cannot elucidate the precise role any of these

meteorological factors would have played in explaining the in-

creased frequency of failures in the 2011 ood, it seems probable

that the above factors would have had a cumulative effect and that

bank erosion rates even in wet ow processes are likely to have oc-

curred episodically both prior to, during, and immediately after, the

ood peak.

6. Conclusions

This study documents the downstream distribution of mass fail-

ures both as a result of a catastrophic ood which occurred in the

Lockyer Valley in January 2011 and those features which formed dur-

ing previous large oods. The features were classied as wet ows

based on some important diagnostics of failure form and processes

and have retained a characteristic morphological shape over time.

These failures are different to the more widely reported gravity af-fected bank collapses. The downstream analysis of these two tempo-

ral distributions revealed the following major conclusions:

• Wet ow failures occur across a range of river lengths, catchment

areas, bank heights and angles and do not appear to be scale-

dependent or spatially restricted to certain downstream zones.

• The downstream trends of each bank failure distribution show limit-

ed spatial overlap.

• Conceptualmodels of downstream process zones may need to consid-

er a wider array of mass failure processes to accommodate for the al-

ternative forms of bankerosion processes such as those reported here.

• The modication of these features during a catastrophicood also in-

dicated that such features tend to form at some ‘optimum’ shape and

show limited evidence for subsequent enlargement even when ow

and energy conditions within the banks and channel were high.

• Such features show evidence for inlling through deposition during

subsequent oods and their identication over time diminishes, al-

though the sharp accurate form of the headwall remains obvious.

It seems clear that such features are an important mechanism for

internal adjustments in channel width during extreme ood events

in the Lockyer Valley. It is also apparent that the increasing availabil-

ity of high-resolution imagery and topographic data sources will con-

tinue to improve our ability to understand the spatial and temporal

distributions of bank failures and provide valuable input to future

bank erosion prediction models.

Acknowledgements

This project was supported by the Queensland's Department of Sci-

ence, Information Technology, Innovation and the Arts (DSITIA) as part

of the Flood Recovery Project 2011 and an Australian Research Council

Linkage Award (LP120200093). Fiona Watson (DSITIARemote Sensing)

provided valuable advice on LiDAR mapping of these features. Phil

Blosch (DSITIA Chemistry Centre) provided access to a programme in

R to calculate channel attributes from LiDAR cross-sections.

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